activeneutrino

Activeneutrino is a term sometimes used to refer to the active neutrinos—the neutrino flavors that participate in the weak nuclear force. In the Standard Model, there are three active neutrino flavors: electron neutrino, muon neutrino, and tau neutrino. They are left-handed and possess extremely small but nonzero masses, which are not yet explained within the minimal Standard Model but are commonly attributed to beyond-Standard Model mechanisms such as the seesaw.

Active neutrinos interact via the weak interaction through both charged-current and neutral-current processes, mediated by W and Z bosons. This allows electron neutrinos to convert to electrons in charged-current interactions, muon neutrinos to produce muons, and tau neutrinos to produce tau leptons; all active neutrinos can scatter off matter through neutral-current interactions.

They are produced in a wide range of processes: in nuclear beta decay, pion and muon decays, the core-collapse supernovae of stars, nuclear reactors, particle accelerators, and cosmic ray interactions. They are detected by observing the products of their weak interactions, such as charged leptons from charged-current events or recoil from elastic scattering off electrons or nuclei, in detectors like Super-Kamiokande, SNO, KamLAND, Borexino, Daya Bay, and other large-volume neutrino observatories.

A key property is neutrino oscillation: as neutrinos propagate, their flavor content changes due to mixing between flavor and mass eigenstates, described by the PMNS matrix. This phenomenon established that neutrinos have mass and implies the existence of at least two nonzero mass differences. Cosmological and laboratory data constrain the number of light active neutrinos to three, while allowing for additional sterile neutrino species if they exist.